1. Field of the Invention
The present invention relates to an optical fiber and a planar waveguide for attenuating received optical signals in an optical communications system.
2. Description of the Related Art
The need for tremendous amount of data to be transmitted in optical communications led the development of Wavelength Division Multiplexing(WDM) in addition to Time Division Multiplexing(TDM). WDM is to transmit a plurality of optical signals having different wavelength through a single transmission line, thus increasing the transmission efficiency of signal.
In the optical communications system, since the signal loss increases with the length of the optical fiber, the distant receiving end receives signals so weakened as to make it difficult to effectively detect them.
In order to resolve the problem of such signal loss, an amplifying means for amplifying the optical signal is disposed between the transmitter and the receiver, and the transmitter fortifies the output signal in order to compensate for such signal loss. However, if a receiving apparatus such as optical fiber amplifier is installed near the transmitter generating signals of high level output, it cannot properly detect such signals. Accordingly, there have been proposed methods of attenuating the optical signal received at the front end of the receiving apparatus. These are to offset the ferules to each other, to cause some amount of light to leak through gaps between the ferules, to make the cores of the optical fiber to have different diameters, or to insert filters between the ferules.
However, the filter-type optical attenuator has the attenuation region too small to precisely control the absorption rate.
It is an object of the present invention to provide an optical attenuator comprising an optical fiber with a core doped with dopants for absorbing light in a particular wavelength band.
It is another object of the present invention to provide an optical fiber for uniformly attenuating an optical signal in a particular wavelength band.
It is still another object of the present invention to provide a planar waveguide for uniformly attenuating an optical signal in a particular wavelength band.
According to an aspect of the present invention, an optical fiber for achieving a substantially uniform optical attenuation comprises a core layer and a cladding layer, wherein the core layer is co-doped with ions of at least one or more of first metals having optical absorption coefficients of negative slope in a particular wavelength band and ions of at least one or more of second metals having an optical absorption coefficients of positive slope in the particular wavelength band.
Preferably, the first metals are Fe, Cr, Mn and V, and the second metals are Co and Ni.
According to another aspect of the present invention, an optical fiber having a core layer and a cladding layer for achieving a substantially uniform optical attenuation comprises a first optical fiber with a core layer doped with ions of first metals having optical absorption coefficients of negative slope in a particular wavelength band; and a second optical fiber with a core layer doped with ions of second metals having an optical absorption coefficients of positive slope in the particular wavelength band, wherein the second fiber is connected with the first optical fiber in series.
According to other aspect of present invention, a planar waveguide for achieving a substantially uniform optical attenuation comprises a core and a cladding layer, wherein the core is co-doped with ions of at least one or more of first metals having optical absorption coefficients of negative slope in a particular wavelength band and ions of at least one or more of second metals having an optical absorption coefficients of positive slope in the particular wavelength band.
Preferably, the first metals are Fe, Cr. Mn and V, and the second metals are Co and Ni.
The above objects and other features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings.